1. Field of the Invention
The present invention relates to the field of increasing throughput in wireless local area network communications.
2. Related Art
A wireless local area network (WLAN) is a transmission system that provides for network access between electronic devices that are wireless end stations using radio waves instead of direct cable connections. In the IEEE 802.11 standard, the WLAN consists of a number of basic service sets (BSS) that are joined by a distribution system into an extended service set (ESS). Within the ESS, a mobile unit or end station may roam at will while continuously maintaining a connection to the network.
Each BSS is controlled by an access point (AP). Each AP communicates with the end stations over the wireless medium in its BSS. The AP communicates with other APs and other nodes on the network via the distribution system. A function of the AP, among many others, is to relay network traffic from the end stations in its BSS through the distribution system to the destination. The destination of this traffic may be another end station in the same, or different, BSS, or the destination may be a node on a wired LAN (such as ethernet) connected to the distribution system. The AP provides this relaying function for multiple mobile units simultaneously. The relaying of traffic for multiple end stations results in an asymmetric distribution of the load entering a BSS.
The IEEE 802.11 standard uses a default or basic access mechanism implemented in the 802.11 Media Access Control (MAC) layer. The 802.11 MAC layer protocol is called the Distributed Coordination Function (hereinafter referred to as “DCF”), that provides fair access to all users of the WLAN.
For example, in a BSS where the AP is relaying traffic for nine end stations, and where the traffic from each end station generates an equal amount of returned traffic to that end station, the IEEE 802.11 standard provides for the default DCF access mechanism to provide fair access to all users, including the AP, of the WLAN.
In the foregoing example, the available bandwidth of the BSS would be shared equally among the nine end stations and the AP, with each approximately receiving a minimum of ten percent of the available bandwidth. In a sense, there is symmetric access to the network, where none of the end stations nor the AP have network access priority over the other users.
Basically when using the DCF access mechanism, a station that senses that the transmission medium is available is allowed to transmit over the WLAN. If the medium is not free, then the station waits for a certain time before trying to transmit again. This waiting period is a called a backoff period. The IEEE 802.11 standard and its variations uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) mechanism with a random backoff for wireless connectivity between fixed, portable, and moving stations within a local area.
Furthermore, the backoff period is a delay of a random length. The backoff period is determined whenever a station detects that the medium is busy at the time the station attempts to begin a transmission, or if it determines that a collision event has occurred because its transmission is not acknowledged according to the protocol operating rules. The backoff period is also used after each successful transmission. This delay is randomly selected from a “contention window” that begins with a fixed value. For each subsequent collision event or busy condition of the medium that is detected for a given or subsequent attempted transmission, the contention window size is approximately doubled and a new random delay is selected from the contention window. In the 802.11b standard, the initial value of the contention window is 31 slots. The sequence of contention window values is as follows: 31, 63, 127, 255, 511, to a maximum of 1023 slots.
As a compromise between higher performance when the network is lightly loaded and minimizing the number of consecutive collisions as the load increases, the initial value of the contention window is relatively large. The result of having a large initial value for the contention window is that transmissions are delayed significantly after determining the medium is busy, suffering a collision event, or on a transmission attempt that immediately follows a successful transmission by the same station. While this is the desired behavior when the network is operating with a significant offered load, in lightly loaded conditions the delay is excessive and results in reduced overall throughput. Thus, a need exists for increasing throughput in a wireless local area network especially during lightly loaded conditions.